Electric control circuit



June '10, 1947. KINGSMlLL 2,422,020

ELECTRIC CONTROL CIRCUIT 42 V 80! V ol. I

Fig. 2.

Sol/[Ill H547 CM l/[ll' 901. #547 l/0L0 RELAY 5o RELAYZJ RELAY 24 RL4Y26 RELAY 27 TIMED BY CAPACITOR 86 77 6'9 6'5 Inventor:

Robert E. fiinbg smiil,

His Attorney.

June 10, 1947. R. E. KINGSMILL 2,422,020

ELECTRIC CONTROL CIRCUIT- Filed larch 27, 1945 2 Sheets-Sheet 2 Fig. lb.

' Inventor: Robert E. Kingsmill,

His Attorney Patented June 10, 1947 ELECTRIC CONTROL CIRCUIT Robert E. Kingsmill, Schenectady, N. Y., assignor to General Electric Com New York pany, a corporation oi Application March 27-, 1945, Serial No. 585,100 14 Claims. (Cl. 171-97) My invention relates to electric control circuits and more particularly to improved control circuits for effecting a plurality oi circuit controlling operations in timed sequence.

While not limited thereto, my invention is particularly applicable to the control of resistance welding systems in which it is desirable to transmit current to a load circuit intermittently for each weld period in a spot welding operation, or for transmitting a single pulse of predetermined duration for each weld. Many timers have been designed for operations of this character but considerable difllculty has been encountered in securing consistent timing operation regardless 01! supply line voltage fluctuations. Some of the controls have been unduly complicated considering the number and flexibility of the timing functions that could be performed. Accordingly, it is an important purpose of my invention to provide for timing one or more operations new and improved control circuits which are reasonably simple and which are consistent in operation with supply line voltage fluctuations of com siderable magnitude.

It is an object of my invention to provide a new and improved electric control circuit.

It is another object of my invention to provide a new and improved electronic timing circuit.

It is a still further object of my invention toprovide for timing a plurality of functions a new and improved electronic circuit which operates consistently independently of supply line voltage fluctuations.

In accordance with the illustrated embodiment of my invention, a plurality of timers, each including an electric discharge device, are employed for energizing electromagnetic relays in accordance with a predetermined time pattern. The anode-cathode circuits of the electric valves and the operating coils of the relays are energized from an unregulated alternating current supply circuit. The circuit of the control members of these electric valves is energized from a regulated direct current bus, the voltage or which is maintained in a suitable manner such as by a regulating gaseous discharge valve having a constant voltage characteristic. With this arrangement, the demand on the direct current system is small even when a large number of timers is employed and the supply of closely regulated direct current voltage is not difficult. In the particular embodiment illustrated, the. timer is designed to control the application of pressure and current to the electrodes of a resistance welder, in a sequence which is well known in the art, including 2 a squeeze period, an overall weld period durin which the welder may be alternately energized and deenergized during heat and cool periods, a hold period during which pressure is maintained on the work and an oil. period during which the work is released with pressure and current both having been terminated. A particularly effective and simple control is employed for eifecting the timing of the heat and cool portions of the weld period.

My invention will be better understood by reference to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. In the drawings, Figs. 1a and 11) considered together as Fig. 1 are a schematic representation of one embodiment of my invention; Fig. 2 indicates the operative condition of various elements of the system illustrated in Fig. 1; and Fig. 3 illustrates the operating characteristics of the heat and cool timers of the system of Fig. 1.

Referring now to Fig, 1 of the drawings, I have shown my invention incorporated in a control system for controlling the energization of a load circuit l which, in the interest of simplification, has been designated merely as a welder initiating circuit for controlling the application of current to a welding load. The conductors i may, for example, correspond to conductors 45 and 46 of the system shown in Bivens application Serial No. 530,136, filed April 8, 1944, and assigned to the assignee of the present invention. The energization of a pressure control circuit, which has been illustrated simply as the operating solenoid 2 of a solenoid valve, may be employed to control the pressure applied to the electrodes of the welder in a manner well understood in the art. The control system includes a supply of alternating current 3 which is connected to energize the primary winding 4 of a transformer 5 having secondary windings 6, l, 8 and 9. The secondary winding 9 provides an altematingcurrent bus including conductors i0 and H. The conductor II is connected through an initiating switch l2 to a bus l3 which may be grounded. The winding 6, which is provided with a mldtap, operates in conjunction with a discharge device I4 to provide a full-wave rectifier system and a supply of direct current voltage. The output of the rectifier is filtered by a series resistor l5 and a shunt capacitor l6 and regulated by a circuit including a resistor l1 connected in series with an electric discharge device l8 of the type having a critical minimum ionization voltage and which operates to draw variable amounts of current through the resistors l5 and H to maintain a constant voltage across the conductors l8 and which are connected to the electrodes of the discharge device l8 and which provide a regulated direct current bus. The direct current voltage of conductor l3, which is also an alternating current supply, is maintained at a voltage between conductors l9 and 20 by voltage dividing resistors 21 and 22. Thus conductors l0 and I3 provide a supply of alternating voltage and conductors l3, l9 and 20 provide a supply of regulated direct current voltage for the energization of the various components of the control system.

In order to effect a plurality of operations in timed sequence in accordance with the desired timed pattern, 2 provide a plurality of electromagnetic relays 23-21, inclusive, each of which i provided with an operating coil 28 connected in series with the anode-cathode circuits, respectively, of electric discharge devices 29-33, inclusive, and across the alternating current supply conductors l8 and An additional electric valve also energized from the alternating current supply conductors ill and i3, is provided for controlling electric valve 38. Each of the electric discharge devices 29-34 is preferably of the type employing an ionizable medium, such as a gas or vapor, and each includes an anode 35, a cathode 33, a control member or grid 37 and a shield grid The control member 31 of each of the valves is connected with its cathode through a transient suppressing capacitor and the shield grid of each of the valves 33, inclusive, is connected directly with its cathode. Since electric valves 29-33 conduct current during halif cycles of voltage of only one polarity, an energy storage circuit in parallel with the operating coils 28 of th electromagnets IS-2i is provided to smooth out the current flow through the coils and, as illustrated, each of the operating coils is shunted by a series circuit including a capacitor 40 and a resistor 4|. As illustrated in the drawings, the heater elements associated with the cathodes of electric valve l4 and electric valves 29-33, inclusive, are all connected in parallel and energized by the voltage appearing across conductor l3 and a second conductor 42, this voltage being supplied by the winding 1 of transformer 3.

The circuits for energizing the control members of electric valves 29-33, inclusive, are energized from the regulated source of direct current voltage including conductors l3, l9 and 20 and are in general similar. However, sinc each control circuit includes the contacts of different relays, in order to effect the desired sequential operation, these circuits will be considered individually. The control member 31 of electric valve 29 is connected to the direct current supply conductor l9 through current limiting grid resistor 43, a charging resistor 44 and a timing capacitor 45. Resistor 44 and timing capacitor 45 are shunted by a range resistor 45 and an adjustable timing resistor 41. Resistor 44 functions to determine the charging rate of capacitor 45 from the voltage of direct current supply conductors l9 and 20 and is connected to the conductor 20 through conductor 48, a normally closed contact 49 of a control relay 50 and conductor 20. In a similar manner, the circuit of the control member of electric valve 3i is connected through a current limiting grid resistor 52, a, charging resistor 53, and a timing capacitor 54 to conductor I9. The common terminal of resistors 52 and 53 is connected with the direct current supply conductor 20 by conductor 55, and a normally closed contact 56 on relay 23. A discharge circuit for timing capacitor 53 is provided by series connected resistors 51, 58 and 53 which are connected in shunt,with the capacitor 53 and resistor 54. Resistor 58 is adjustable for adjusting the discharge rate of capacitor 54, and a manually operable switch 50 is arranged to short circuit resistor 51 to change the range of operation of the timer. A second manually operable switch 6| is arranged to shunt both resistors 51 and 58 when in its closed position to eil'ectively connect the control member of electric valve 3| with the direct current line l9 to pick up the relay 25 of the weld timer substantially immediately upon timing out of the squeeze time relay 23 when the system is used for ordinary spot welding. With switch 6| closed, the duration of energization of the load for the spot weld is determined by the operation of the heat timer including electric valve 30 and relay 24, The control member of electric valve 32 of the hold timer is connected to conductor l9 through a current limiting grid resistor 62, charging resistor G3 and timing capacitor 64. The charging circuit for capacitor 64 is completed to the supply conductor 20 through the normally closed contact 65 of relay 26 and a normally closed contact 56 of relay 25. Th circuit to the supply conducto 20 may also be completed through a normally open contact 61 of relay 24 which shunts normally closed contact 56 of the relay 25, A discharge circuit for the timing capacitor 54 is provided by a range resistor 58 and an adjustable timing resistor 69 which are connected in series with one another and in shunt with resistor 53 and capacitor 64. In a similar manner, the conrol member of electric valve 33 is connected with the conductor l9 through a current limiting grid resistor 10, a charging resistor '11 and a timing capacitor 12. The charging circuit of the capacitor I2 is completed to the conductor 20 through conductor 13 and a normally open contact 5| of relay 25. A discharge circuit for the timing capacitor 12 is provided by a range resistor l4 and an adjustable timing resistor 15 connected in series with one another and in shunt with the resistor H and timing capacitor 12. A single pole manually operable switch 16 is connected in the anode-cathode circuit of electric valve 33 to disable this timing circuit which is the off-time timer when non-repeat operation is desired.

Electric valves 30 and 34 and associated control circuits function as heat and cool timers, respectively, when the system is operated for pulsation welding. The anode-cathode circuit of the electric valve 30 has been previously described. The control member 31 of this electric valve is controlled in accordance with the voltage on a capacitor 11 which is connected in a charging circuit across the direct current supply conductors l9 and 20. As illustrated in the drawings, one terminal of the capacitor 11 is connected with conductor IS'and the other terminal is connected through resistor 18 and adjustable resistor 19 to an adjustable point on voltage dividing resistor which is connected across the direct current supply lines l9 and 20. The control member of electric valve 30 is connected with the terminal oi capacitor ll, remote from line l9, through a current limiting resistor 82. The discharge of capacitor I1 is controlled by electric valve 34 which functions with the capacitor 11 to form a control circuit in the nature of a single tube inverter. The

cathode of electric valve 84 is connected with the same terminal oi capacitor I1 as the control member of electric valve so, while the anode of electric valve 84 is connected to the supply line 18 and the other terminal of capacitor l1 through an inductive reactor t3, conductor 84 and a normally closed contact 85 of relay 26. The operation of the electric valve at to control the voltage of capacitor i1 is determined by an energizing circuit for the control member thereof which includes a timing capacitor 36*, which is connected across the direct current supply lines i9 and 20, through a charging resistor ill, conductor 88 and a normally open contact 89 of relay 24. A discharge circuit for capacitor so is provided by a range resistor 98 and an adjustable timing resistor 9i connected in series with one another and in shunt with the capacitor and resistor 87. order that the operation of the electric valve 34 may be initiated the control of the relay 23, the control grid reel? is connected through cnrre limiting grid resistor S2 to the direct can out line 28 through a normally closed contact 93 of relay 23 to the lower terminal of charging resistor 81 througha normally open contact 94 of relay 23. With this arrangement, the grid of electric valve 34 is connected to a negative voltage as long as relay 23 is deenergized and is connected for energization in accordance with the charge on timing capacitor 86 upon energization of relay 23. In order to completely discharge the timing capacitor 86 at the end of the weld interval to insure immediate ilring of tube M at the end of the squeeze time oi the next sequence, I provide a circuit for rt circuiting the timing capacitor through recr ill and including conductor 95, a normally till ill on relay 25. The shield grid oi elecvalve 38 is connected at a voltage slightly ativc with respect to the direct current supply e provided by a voltage divider including istors 9B and d9 connected in series hetween doctors 83 and to. This connection functions nit the negative voltage to which lower ninal of capacitor ll charges through resistors and l9 as will become more apparent from the detailed description of operation of the system which follows in a later part of this specification. Since in operation the cathode of electric valve tit is at variable potential, the cathode heater energized by a separate source of heating current provided by transformer winding s.

- As stated earlier in the specification, the control relay 58 controls the initiation or operation of the squeeze timer. This rela 50 is provided with an operating coil I which is connected to be energized from the alternating current supply lines Ill and i3 through a circuit including a normally closed contact ill! of relay 26. It will be understood that the alternating current voltage is impressed on conductor i3 by closure of the initiating switch i2 so that the control relay 50 is energized as soon as the initiating switch is closed. In addition to the normally closed contact 48 previously described, the relay 50 is provided with normally open seal-in contacts I02 and normally open control contacts I03 which -control the operating coil of the solenoid valve 2 from a source oi alternating current I04. pressure is applied to the welding electrodes (not shown) during the interval that the control relay 50 is energized. The relay 21 of the oil-time timer includes a normally closed contact I05 connected in the anode-cathode circuit of electric valve 32 which is a part of the hold timer. When sad contact on relay 2t and a normally open the system is adjusted for repeat operation, relay 2! opens contact it! to deenergize relay 28 and reclose control relay III to reinitiate the sequence of timing operations. I

A better understanding 01' my invention may be had by a brief consideration of the operation of the illustrated embodiment thereof. It will be assumed that the transformer 3 is energized, that all of the cathodes are at proper operating temperature, and that manual switches 50 and ti are open and manual switch 18 is closed. This adjusts the system for pulsation welding with repeat operation as long as initiating switch it is retained closed.

in Fig. 2. the operative condition of the relays, initiating switch l2, and tube 34 are illustrated for all intervals oi the sequence of operations. solid line indicates that the relay is energized and a dotted line indicates that the timing circult controlling the relay has been initiated in operation.

M soon as switch i2 is closed, the operating coil :00 of relay 50 is energimd to close contacts i027 and I03 and to open contact 49. ilontacts Hi2 seal in the energizing circuit for the operating coil Hill of relay 50. Contacts m3 energize the operating coil of the solenoid valve 2 to effact the application of pressure to the welding electrodes (not shown). Opening of contact 49 disconnects the capacitor 45 from the direct cur-= rent line. 28 and the lower plate of the timing capacitor discharges through a circuit including resistors 44, 46 and 41. After a predetermined interval of time, the control member of electric valve 29 becames suihciently positive to initiate conduction of valve 2s and energize relay 23 to terminate the first interval of the operation or squeeze time during which pressure only is ap plied to the electrodes.

Energization of the relay 23 opens contacts 56 and 93 and closes contact st. Upening of contact 58 disconnects the timing capacitor 54 of the weld timer from the negative direct current bus 20 to initiate timing of the weld interval. Opening of contact 93 and closure of contact at transfers the control grid of electric valve 3c from the negative direct current conductor 20 to the lower terminal of resistor iii". Since capacitor 86 has been maintained discharged, by

reason of its charging circuit being open at con" 85 of relay 24 of the heat timer, electric valve M is immediately rendered conductive and the capacitor 11, which has previously been charged through a circuit including resistors l8 and 79 to a predetermined voltage, is discharged through the anode-cathode circuit of electric valve 34 and the reactor 83. The reactance in the discharge circuit efl'ects a, reversal of voltage of the capacitor 1! so that the lower plate of capacitor "i1 is somewhat positive with respect to conductor 89.

The operation of this portion of the circuit may be better understood by reference to Fig. 3

in which electric valve 34 and the control circuit associated therewith have been reproduced and the voltages of capacitors I1 and 86 have been plotted as E" and E respectively. (in curve E11 condition of electric valve 34 is initiated at point a and terminates at point D. After electric valve 34 has been extinguished at point D, the voltage of capacitor 11 decays on an exponential which is tangential to the voltage determined by the slider on resistor 80. However, when the voltage reaches a value corresponding to the voltage of the common terminal or resistors 88 and M, the shield grid circuit of electric valve 14 draws current through a circuit including resistors 18 (shown only in Fig. l) and 18 to maintain the voltage of capacitor 11 substantially constant. As the voltage of the lower terminal of capacitor ll rises on the curve Ell between points a and b, the control member oi electric valve so is raised to a suilficiently positive voltage to render electric valve 30 conductive. Conduction by this valve energizes relay 24 of the heat timer to close the normally open contact 24 in the welder initiating circuit. Operation of relay 24 also closes contact 89 to complete a charging circuit for timing capacitor 88 to charge the lower terminal thereof to the voltage of conductor 20. The negative voltage oi conductor 20, which is applied to the control element of valve 34, also holds this valve nonconductive after its discharge of capacitor ll. When the voltage of capacitor H has decayed along the curve Ell from point '0 to a point indicated at c, the electric valve 30 becomes nonconducting and the relay 24 drops out terminating a period or interval of energization of the load circuit by opening contacts 24.

Deenergiaation of relay 24 opens contact 89 to initiate a timing operation by timing capacitor 88 which is disconnected from conductor 20 and which discharges through a circuit including resistors 81, 80 (shown only in Fig. l) and 9|. When the voltage of the lower plate of capacitor it rises suiliciently to render the control member of electric valve 34 less negative with respect to the cathode than the control voltage, as indicated at point d, electric valve 34 is again rendered conducting to discharge the capacitor 11 which applies to the control element of valve 30 the positive voltage value indicated at e from which it charges to the point I to establish the interval during which electric valve 30 conducts.

In this manner, electric valve 30 is conductive and non-conductive alternately for intervals determined, respectively, by the timing capacitor l1 and its associated circuit and the timing capacitor 22 and its associated circuit. This operation continues for an interval following energization of the relay 23 of the squeeze period timer until energization of the relay 25 of the weld interval timer. When relay 25 picks up at an interval determined by the timing capacitor 54 and its discharge circuit including resistors 53, 51, 52 and 59, contact 85 is opened and contact 91 is closed. Opening of contact 85 interrupts the anode-cathode circuit of electric valve 34 to prevent further discharging of capacitor 11 through this valve and contact 91 establishes a discharge circuit for the timing capicitor 86 after relay 24 drops out closing its control contact 28. Operation of the relay 25 also opens contact It to initiate the timing operation of the hold period control including electric valve 22 provided contact 61 of relay 24 is also open. Operation of relay 25 also closes contact to effect the charging of timing capacitor 12 and condition the off-time timer including electric valve 22 for a timing operation by rendering electric valve 32 nonconducting and dropping out relay 21. When initiating switch l2 was closed, contact ll of relay 25 was open and there was no charge on capacitor 12, 50 valve :1 became conductive, energizing relay 21.

Aiter an interval oi time determined by the discharge of capacitor 64 through resistors 82, 68 and 40, the relay 26 of the hold timer picks up opening contacts 04 and Ill. The opening of contact llll deenergizes the control relay II to reset timing relays 23 and 25 and the opening oi contact initiates the timing operation of the ctr-time timing circuit and maintains valve 22 conductive while the other timing circuits are reset. After the expiration of an interval of time depending upon discharge of capacitor 12 through resistors ll, 14 and I5, and ii switch i2 is still depressed, relay 21 oi the oft-time timer is energized and opens its contacts I" to drop out relay 28 of the hold-time timer which closes its contacts HH, and the sequence or operations described is reinitiated.

In the foregoing description, the operation oi the system for pulsation welding has been considered. Ii switch 8| is closed, the relay 2! oi the weld timer is energized substantially immediately upon operation of relay 23 0! the squeeze timer with the resultant opening 01 its contact 56. With this sequence of operation, the duration of the weld current is established by a single operation 01' the relay 24 oi the heat period timer. The contact 61 of the relay 24 of the heat period timer shunts the contact B0 of the weld interval timer to permit the completion of a full heat period once it is initiated, regardless of energization of the weld interval timer.

From the description of the operation oi the control relay 50 earlier in the specification, it is apparent that when the system is adjusted ior non-repeat operation by opening manually operable switch 16, it is only necessary to momentarily close the initiating switch I2. As soon as relay ill is energized and sealed in through its holding contacts I02, the switch l2 may be released and the operation proceeds until the relay 26 of the hold-time timer is energized, dropping out the control relay 50 and locking the system against further operation until subsequent closure of initiating switch I2 again energizes bus II from the alternating current source oi supply. With the system described, it is believed apparent that the duration oi the heat and cool periods may be established independently by adjustment of resistors 18 and 8!, respectively, and that the intermittent energlzation of the load for pulsation welding is ei'iected with a minimum of equipment. Also, by the utilization of separate direct current and alternating current supply lines for the control circuits and anode-cathode circuits, respectively, of a group of cascaded electronic timers, it is possible to provide consistent operation regardless of supply line voltage fluctuations without encountering an unreasonable expense in regulating a direct current bus of large capacity.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without. departing from my invention in its broader aspects, and I. therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent oi the United States is:

1. In combination, a source or direct current voltage including means for establishing three points each at a different level of voltage, a resistor and a capacitor connected in parallel and having a common terminal connected to a point of extreme voltage, means including a normally closed initiating switch connecting the other common terminal oi said resistor and capacitor 15 to the other point of extreme volts-8e. an electric valve including an anode, a cathode and a conof extreme voltage, an electric valve including an anode, a cathode and a control member, means connecting the cathode of said electric valve to said point of intermediate voltage, and means connecting said control member for energization in response to the voltage terminal of said cai pacitor electrically nearer said initiating switch.

3. In combination, a source of direct current voltage including voltage dividing means for establishing different levels of voltage, an electric valve including an anode, a cathode and a con trol member, means connecting the cathode of said electric valve for energization in accordance with the voltage of an intermediate point of said voltage dividing means, a resistor and a capacitor connected in parallel and having a common terminal connected to a point of said dividing means which is positive with respect to said in termediate point, means including a normally closed initiating switch connecting the other common terminal of said resistor and capacitor to a point of said dividing means which is nega tive with respect tosaid intermediate point, and means connecting said control member for energization in accordance with the voltage of said capacitor.

4. In combination, a source of direct current including three terminals and means for establishing on said terminals three levels or direct current voltage, source of alternating current, on electric valve including an anode, a cathode and a control member, means connecting one line of said alternating current source with the terminal of intermediate direct current voltage, means connecting the anode-cathode circuit of said electric valve across said alternating cursource, a circuit including a resistance and capacitance energized from the remaining teror said source of direct current, and means connect-"11g the control member of said electric valve energization in accordance with the voltage of capacitance.

5. In combination, a source of direct current including three terminals and means for-establishing on said terminals three levels of direct current voltage, a source of alternating current, an electric valve including an anode, a cathode and a control member, means connecting one line of said alternating current source with the terminal of intermediate direct current voltage, means connecting the anodecathode circuit of said electric valve across said alternating current source, a circuit including a resistance and a capacitance permanently connected with the more positive of the remaining terminals of said source of direct current, means including an initiating control means connecting said resistance and said capacitance with the remaining terminal of said source of direct current, and means connecting the control member of said 10 electric valve for energization in accordance with the voltage or said capacitance.

6. In combination, a source of direct current including three terminals and means for establishing on said terminals three levels or direct current voltage, a source 01' alternating current, an electric valve including an anode, a cathode and a, control member, means connecting one line of said alternating current source with the terminal 01' intermediate direct current voltage, means connecting the anode-cathode circuit of said electric valve across said alternating current source, a circuit including a capacitor energized from the remaining terminals of said direct current source, circuit controlling means con-- nected between said capacitor and the more negative of said remaining terminals ior controlling the charging of said capacitor from. said remaining terminals, a discharge circuit shunting said capacitor, and means connecting the controi member of said electric valve for energization in response to the voltage of the terminal of said capacitor eiectrically nearer said circuit controlling means.

'1. In combination, a source of regulated direct current voltage, a source of alternating current voltage, a plurality of electronic timers each including an electric valve having an anode, a cathod and a control member, a plurality of electromagnetic relays each including an. operating coil, means connecting th'e anode-cathode circuit of each of said electric valves in series with the operating coil of a different one of said relays and for energization from said source of alternating current voltage, a timing circuit connected with each of said control members and each comprising a capacitor and charging and discharging circuits therefor, means connecting each of said capacitors for energization from said source of regulated direct current voltage, and means for initiating a transient change in voltage on each of said capacitors in. sequence to render the associated electric valves conducting in seduence at intervals determined by said transient voltages.

8. in combination, a source direct current voltage, a source cl alternating current voltage, a plurality oi. eiectronic timers each including an electric valve having an anode, a cathode and a control member, a plurality of electromagnetic relays each including an operating coil, means connecting the anode-cathode circuit of each of said electric valves in series with the operating coil of a diiferent one of said relays and for energization from said source of alternating current voltage, a timing circuit connected with each of said control members and comprising a capacitor and a resistor, means normally connecting each of said capacitors for energization from said source of direct current voltage to establish a predetermined charge thereon, means establishing discharge circuits for each of said capacitors, and means interrupting the charging circuits of said capacitors in sequence to render the associated electric valves conducting in sequence at intervals determined by the discharge of the respective capacitors.

9. A timing circuit including a, voltage supply circuit, a capacitor, a charging impedance, means connecting said capacitor and said charging impedance in series and for energization from said supply circuit, an electric valve including an anode, a cathode and a. control member, means connecting the anode-cathode circuit of said valve in parallel circuit relation with said capacitor so that tho voltage oi the cathode of said valve is controlled by the voltage of said capacitor, and means for limiting the voltage to which said capacitor charges to a predetermined value which is less than the voltage impressed across said capacitor and charging impedance by said supply circuit.

10. A timing circuit including a voltage supply circuit, a capacitor, a charging impedance, means connecting said capacitor and said charging impedance in series and for energization from said supply circuit, an electric valve including an anode, a cathode and a control member, means connecting the anode-cathode circuit of said valve in parallel circuit relation with said capacitor so that the voltage of the cathode of said valve is controlled by the voltage of said capacitor, and means including a unilaterally conducting path connected between a point or intermediate voltage on said supply circuit and the more negative terminal of said capacitor for limiting the voltage to which said capacitor charges to a value determined by said point of intermediate voltage.

11. A timing control including a voltage supply circuit, a first capacitor, a charging impedance, means connecting said first capacitor and said charging impedance in series and for energization from said supply circuit, an electric valve including an anode, a cathode and a control member, means connecting the anode-cathode circuit of said valve in parallel circuit relation with said capacitor so that the voltage of the cathode of said valve is controlled by the voltage of said capacitor, a timing circuit including a second capacitor for controlling the energlzation of said control memb-er, means responsive to the voltage of said first capacitor for effecting the connection of said second capacitor across said supply circuit and its disconnection therefrom, and means for limiting the voltage to which said first capacitor charges to a value which is greater than the voltage required for effecting the disconnecting operation of said last mentioned means but less than the voltage impressed across said first capacitor and charging impedance by said supply circuit.

12. A timing control including a direct ourrent voltage supply circuit, a first capacitor, 9.

charging impedance, means connecting said first capacitor and said charging impedance in series and for energization from said supply circuit, an electric valve including an anode, a cathode and a control member, means connectin the anode-cathode circuit or said valve in parallel circuit relation with said capacitor so that the vol e or the cathode of said valve is controlled by the voltage or said capacitor, a timing circuit including a second capacitor for controlling the energization of said control member, means responsive to the voltage of said first capacitor for initiating a transient voltage change on said second capacitor to vary the voltage of the control member of said electric valve, and means for establishing a voltage to which said first capacitor charges which is independent of the voltage impressed across said first capacitor and charging impedance by said supply circuit.

13. In combination, a source of direct current voltage, a capacitor, a charging circuit for said capacitor including a charging impedance, means for energizing said charging circuit with a variable voltage from said source, an electric valve and inductance means connected in parallel circuit relation with said capacitor, means for rendering said electric valve conducting periodically to discharge said capacitor, and means for 1imiting the voltage to which said capacitor is charged from said direct current source independently of the voltage impressed on the charging circuit, said means including an electrode of said electric valve and a connection between said electrode and a voltage having a fixed relation with respect to the voltage of said source.

14. In combination, a direct current voltage supply circuit, a capacitor, a. charging circuit for said capacitor including a charging impedance, means for energizing said charging circuit with a' variable voltage from said supply circuit, an electric valve and inductance means connected in parallel circuit relation with said capacitor so that the cathode voltage of said valve is determined by the voltage oi' said capacitor, means responsive to the attainment of a predetermined voltage by said capacitor for rendering said electric valve conducting to invert the voltage on said capacitor, and means for limiting the voltage to which said capacitor is charged from said direct current circuit independently of the magnitude of said variable voltage, said means including a unilaterally conducting path connected between said capacitor and a voltage having a fixed relation with respect to the voltage or said supply circuit.

ROBERT E. KINGSMILL 

